A dream of neuroscience is to be able to non-invasively modulate any given region of the human brain with high spatial resolution. This would open new horizons for understanding human brain function and connectivity, and create completely new options for the non-invasive treatment of brain diseases such as intractable epilepsy, depression, and Parkinson's disease. Current non-invasive brain stimulation methods such as transcranial magnetic stimulation (TMS) and transcranial electrical stimulation (TES) can be applied only to superficial cortical areas, with crude 1 cm-scale resolution, limits placed upon these techniques by fundamental physics. Ultrasonic neuromodulation, the use of ultrasound as an energy modality to affect the activity of the brain, could overcome these limitations and thereby transform both basic and clinical human neuroscience. In fact, the engineering challenge of non-invasively focusing ultrasound to mm-sized regions, either shallow or deep in the brain, has been solved: clinical studies have already demonstrated the feasibility of making focal (~ 3 mm diameter) brain lesions in subcortical regions through transcranial high intensity ultrasound. Furthermore, recent human studies have documented enhanced sensory discrimination following relatively mild ultrasound stimulation. These two findings suggest that ultrasonic neuromodulation has the potential to serve as a game-changing new tool for functional dissection of the human brain, and development of non-invasive therapies for human brain disorders. However, we believe three major questions need to be addressed before ultrasound can be used as an effective and safe tool for modulating human brain activity: (1) What are the basic biophysical mechanisms through which ultrasound acts to affect neural activity? (2) What are the optimal ultrasound parameters for maximally modulating neural activity in the primate brain? (3) How does ultrasound targeted to specific brain areas affect the spatiotemporal pattern of activity across the entire brain to causally modify behavior? We will address these three fundamental questions through a systematic effort spanning in vitro preparations, rodents, macaques, and human subjects. First, we will elucidate the endogenous mechanisms by which ultrasound produces changes in neural activity through biophysical experiments in oocytes, purified lipid bilayers, and cell cultures (Shapiro). Second, we will identify the optimal parameters for eliciting ultrasonic neuromodulation in the macaque, the closest animal model of the human brain, through EEG, fMRI, and single-unit recordings (Tsao). Finally, following initial macaque studies, we will test the effects of ultrasound stimulation on te human brain, both spatially through fMRI (O'Doherty) and temporally through EEG (Makeig), examining effects both during rest and during performance of decision-making tasks. The innovations this project will provide are exactly those called for by RFA-MH-14-217: development of breakthrough technology to measure brain processes that were formerly inaccessible to imaging, including...local and micro-circuits in the nervous system and mechanisms linking single cell or circuit activity to hemodynamic or macro-electromagnetic signals. Ultimately it's the combination of local circuit perturbation with non-invasive imaging that will give us the greatest insights into brain function. The pairing of focal ultrasound with fMRI/EEG has potential to reveal human brain circuits with unprecedented spatial resolution and create a new bridge for linking circuit activity to non -invasively measured brain signals. Our approach is only possible through intense collaboration among a unique multidisciplinary team working across model systems, and prepares the necessary experimental foundations to test whether ultrasound is the answer to the long -held dream for a technique to focally stimulate any part of the human brain at will.